Generally, a silver halide emulsion is prepared by a step involving the formation of silver halide grains by double decomposition of a soluble silver salt with a soluble halide in an aqueous gelatin solution, followed by a physical ripening step, a desalting step, and a chemical ripening (after-ripening or second ripening) step of the resulting emulsion.
A spectral sensitizing dye is, in general, added to a photographic emulsion which has already been chemically sensitized before it is applied to a substrate. In U.S. Pat. No. 4,425,426, however, methods involving adding a spectral sensitizing dye to a photographic emulsion before the beginning of, or in the course of, chemical sensitization have been disclosed. Further, in U.S. Pat. Nos. 2,735,766, 3,628,960, 4,183,756, and 4,225,666, methods involving adding a spectral sensitizing dye to a photographic emulsion before completion of the formation of silver halide grains have been disclosed.
More particularly, U.S. Pat. Nos. 4,183,756 and 4,225,666 disclosed that a method involving adding a spectral sensitizing dye to a photographic emulsion after formation of stable nuclei, in particular until adding 85% by weight of a silver salt solution, during the formation step of silver halide grains had advantages, such as an increase in photographic sensitivity and strengthened adsorption of the spectral sensitizing dye by silver halide grains. However, such an addition method is not only troublesome to carry out, but also presents problems in that it changes significantly the grain size distribution and the form of the silver halide grains formed, and damages the photographic performance of the emulsion.